Spark-discharge apparatus with ground detection safety circuit

Abstract

A spark discharge test apparatus has a holder electrode conductively engageable with a workpiece, a counter electrode adapted to be spaced by a gap from the workpiece while it is in contact with the holder electrode, a power supply including a closable switch and connected across the electrode for passing a unipolar electrical pulse across the gap only when the switch is closed, and a circuit including a terminal connectable to the workpiece and connected so as to close the switch allowing operation of the power supply only on detection of a closed circuit between the terminal and the holder electrode. Thus, a unipolar electrical pulse can only be fired between the counter electrode and the workpiece when the workpiece is in good electrical contact with the holder electrode. This safety circuit comprises a low-voltage relay having one side connected to a low-voltage power supply and another side connected to the terminal. The other side of the low-voltage power supply and the holder electrode are connected to ground.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims: 
     
       1. A spark discharge apparatus for testing the composition of a conductive workpiece, said apparatus comprising a holder electrode conductively engageable with said workpiece; a counter electrode adapted to be spaced by a gap from said workpiece while same is in contact with said holder electrode; sparking means including a closable switch and connected across said electrodes for passing a unipolar electrical pulse across said gap only when said switch is closed; and circuit means including a terminal connectable to said workpiece and an electrical connection to said holder electrode, said circuit means being connected to said switch for closing same only on detection of a closed circuit between said terminal and said holder electrode, whereby said sparking means can only pass said pulse when said terminal is in electrical contact with said workpiece and said workpiece is in electrical contact with said holder electrode and by this the operator and the workpiece are on the same potential. 
     
     
       2. The apparatus defined in claim 1 wherein said sparking means includes a high-voltage power supply having one side connected to said holder electrode and another side connected to said switch, the switch being connected to said counter electrode to connect said high-voltage power supply across said gap when said switch is closed, said circuit means including a low-voltage power supply having a loop including said terminal, a coil operatively connected to said switch to close same on closing of said loop, and said holder electrode. 
     
     
       3. The apparatus defined in claim 2 wherein said circuit means includes a second loop independent of the first-mentioned loop and including a second power supply, a second normally open switch directly operated by the first-mentioned coil, and a second coil in series with said second switch across said second power supply, said first switch being directly operated by said second coil, whereby passage of current through said first coil closes said second switch and permits said second coil to be energized by said second power supply and close said first switch. 
     
     
       4. The apparatus defined in claim 3 wherein said circuit means includes a third loop independent of said first and second loops and including a power supply and in series a third normally closed switch directly operated by said second coil, a starting switch, and a third coil, said second loop further having a third normally open switch in series with said second coil and said second switch, said third switch being directly operated by said third coil, whereby closing of said starting switch can energize said third coil and close said third switch to allow energization of said second coil. 
     
     
       5. The apparatus defined in claim 4 wherein said second loop further has in series with said second coil a normally open timing switch. 
     
     
       6. The apparatus defined in claim 4 wherein said third loop includes a condenser connected across said third coil for maintaining same energized for a predetermined period even after said starting switch or said normally closed switch has opened. 
     
     
       7. The apparatus defined in claim 4, further comprising a normally open fourth switch connected in parallel across said third switch and means for detecting current flow through said first loop and for maintaining said fourth switch closed so long as such flow is detected. 
     
     
       8. The apparatus defined in claim 4 wherein said first loop includes a diode and a variable resistor across said diode. 
     
     
       9. The apparatus defined in claim 4 wherein said first loop includes an induction coil in series with said gap. 
     
     
       10. The apparatus defined in claim 1 wherein said holder electrode is a base adapted to support said workpiece.